At the beginning of the solid-stating process, PET pellets are crystallized usually with hot air or in mechanically-mixed, hot-oil-heated vessel. Building molecular weight in the solid-state requires extensive crystallization and/or annealing so that pellets will not stick as they enter the solid-stating reactor at typically 195 to 220° C. Polyester (or copolyester) pellets are generally supplied to converters in a semi-crystalline form. Converters desire to process semi-crystalline pellets rather than amorphous pellets because the semi-crystalline pellets can be dried at higher temperatures without agglomerating. Drying the pellets immediately prior to extrusion of the melt to make bottle performs is necessary to prevent hydrolytic degradation and loss of intrinsic viscosity (It.V.) of the melt inside the extruder. However, drying amorphous polyester pellets at or above the Tg of PET without first crystallizing the pellets will cause the pellets to agglomerate at higher temperatures (140° C. to 180° C.) in the dryers. Feeding amorphous pellets to an extruder will cause the screw to be wrapped as the pellets become hot enough to crystallize in the extrusion zone.
From the pellet manufacturing side, a typical commercial process involves forming the polyester polymer via melt phase polymerizing up to an It.V. ranging from about 0.5 to 0.70, extruding the melt into strands, quenching the strands, cutting the cooled polymer strands into solid amorphous pellets, heating the solid pellets to above their Tg and then crystallizing (also known as crystallization from the glass since the pellets to be crystallized start at a temperature below their Tg), and then heating the pellets in the solid state to an even higher temperature while under nitrogen purge (or vacuum) in order to continue to build molecular weight or It.V. (i.e. solid stating). The solid stating process runs hot enough to make it necessary to first crystallize the pellets to prevent agglomeration at the solid stating temperatures. Thus, crystallization is necessary to avoid agglomeration of the pellets during solid stating and during the drying step prior to extruding the melt into bottle performs.
Typical melt phase polyester reactors produce only amorphous pellets. To make these pellets crystalline, they are usually heated to elevated temperatures in a crystallization vessel while being constantly stirred using paddles or other mechanical rotary mixing means in order to prevent sticking or clumping in the crystallization vessel. The crystallizer is nothing more that a heated vessel with a series of paddles or agitator blades to keep the pellets stirred (e.g. Hosakawa Bepex Horizontal Paddle Dryer). Rotary mixing means suffer the disadvantage of requiring additional energy for mechanical rotational movement, and rotational mechanical agitation required to keep the pellets from sticking can also cause chipping and other damage to the pellets, leading to dust generation or the presence of “fines” in the crystallizer and product. These small pieces of chipped off plastic can often cause extrusion problems if not properly removed.
Alternately, a crystallizer can consist of injecting hot gas into a vessel known as a hot, fluidized mixed bed, mostly containing already crystallized pellets which prevents the amorphous pellets being fed to the vessel from sticking to one another (e.g. a Buhler precrystallizer spout bed unit). Such commercial processes utilize the “thermal” crystallization technique by employing a hot gas, such as steam, air, or nitrogen as the heating medium. The residence time in hot fluidized mixed bed processes is up to six hours. These processes also suffer the disadvantage in that large quantities of gas are required, requiring large blowers and making the processes energy intensive.
Each of these crystallization processes is rather slow and energy-intensive. Crystallization processes can take up to six hours, require energy to turn mechanical rotary mixing means in some cases, have high energy requirements to process hot gases or oil, and the pellets are usually cooled from the pelletizer to about 25 to 35° C. after which they are reheated prior to and during crystallization. Moreover, crystallization vessels are fed with low It.V. pellets suitable, which in turn are solid stated into higher It.V. pellets required for making a suitable bottle. It would be desirable to crystallize polyester polymers in a more energy efficient manner or in lower cost equipment. For example, it would be desirable to reduce the residence time of the polyester polymer in the crystallizer, or provide a process which avoids the energy requirements of mechanical rotary mixing means or of cooling and reheating between pelletization and crystallization, or which even could avoid the step of solid stating altogether, while providing to the converter a high temperature crystallized pellet to enable the converter to dry the pellets at conventional temperatures (typically at 140° C. to 180° C.). Obtaining any one of these advantages would be desirable.